The present invention relates to adjusting transmit power spectra of both a subscriber device and a central transmit/receive device of a communications network, as it is, e.g. required in DSL technology (DSL=Digital Subscriber Line).
Due to the plurality of pairs of wires in a telephone cable from a central office (CO) to connected subscribers or subscriber devices, respectively, crosstalk occurs despite a direct current insulation of the pairs of wires, both near-end crosstalk (NEXT) and also far-end crosstalk (FEXT). The same result from capacitive or inductive couplings. By twisting the individual pairs of wires of a cable, this crosstalk is minimized so far that it may be neglected in the voice band. In data transmission technology, like e.g. DSL technology, however, apart from the voice band further frequency ranges are used, in which this crosstalk has a very strong effect on the possible range and transmission speed.
Near-end crosstalk means, that a transmitter on the “near side” of a receive device, i.e. both receive device and also transmitter are part of different subscriber terminal devices or both receive device and also transmitter are part of the central office, overcouples transmit signal portions into the receive branch of the receive device.
In contrast to that, far-end crosstalk means, that, in addition to the receive signal determined for the same, sent from a far-end first transmitter, i.e. first transmitter and receiver are on different sides of subscriber and central office side, a receiver receives portions of a second transmit signal sent from a far-end second transmitter, i.e. also second transmitter and receiver are on different sides.
In an xDSL network, wherein “x” stands for different implementations of DSL technology, the subscriber devices (CPE=customer premises equipment) are typically in a different distance from the central office (CO). A so called near-far problem results from this. If all subscriber devices of the network transmitted with the same transmit power spectral density (TX-PSD, transmit power spectral density), the CPEs closer to the CO would respectively couple a high amount of crosstalk into the lines of the CPE further away from the CO and thus cause differently strong interference power spectral densities of the individual subscribers.
For this reason, within the scope of VDSL standardization (VDSL=very high speed digital subscriber line), a so called upstream power back-off (UPBO) is defined. This is a method for determining the transmit power spectral densities (TX-PSDs) of subscribers on pairs of wires of different lengths in a multiple-pair communication table, such that the interference power spectral density caused by each pair of wires comprises the same value due to far-end crosstalk (FEXT-PSD) at the CO.
For performing the currently standardized UPBO method the steps discussed in the following are performed during a training phase during a connection setup between CO and CPE. At the beginning of a training phase, the CO transfers standard signals to be presented to the CPE. The same may for example contain information about a signal power with which signals were transmitted from the CO, default values with regard to a power with which the upstream signals sent from the CPE are to arrive at the CO, and also shaping coefficients (Ai, Bi) for frequency bands i for a frequency-dependent shaping of the spectral transmit power density of the CPE. Using these parameters, in a first step by the CPE power parameters of the physical connecting passage between CO and CPE are determined. Here, for example, at the CPE a receive signal strength is determined and using the information about the original central-side transmit power of the signals from the CPE a measure kl0 for the attenuation is estimated, which the signals experience on the way from the CO to the CPE. Using this knowledge, on the subscriber side a transmit power may be determined such that the signals reach the CO as accurately as possible with the power requested from the CO. Values for this requested receive power or receive power spectral density, respectively (RX-PSD), may for example be given by a network operator for example via network management adjustments. In a further step of the training phase, the attenuation ratio is transmitted from the CPE to the CO.
Currently, the standardized UPBO assumes a collocation of a VDSL-switch either in a VDSL-cabinet or in an exchange. The lines to the VDSL-subscribers, however, can be of different lengths. As aforementioned, the aim of the UPBO is to reduce the TX-PSDs of VDSL-subscribers with shorter lines more than those with longer lines to obtain approximately the same upstream-data rate for all connected VDSL-subscribers, irrespective of the length of the respective subscriber lines.
It is sometimes the case, that in an xDSL network ADSL lines (ADSL=asymmetric digital subscriber line) are provided together with VDSL lines (VDSL=very high speed digital subscriber line) in cable bundles. Due to higher data rates of VDSL systems, VDSL central offices are generally installed closer to the corresponding connected VDSL subscribers than comparable ADSL central offices with regard to their connected ADSL subscribers. The lines respectively starting from the central offices are combined in cable bundles and led in parallel at the end of the subscriber side for a few hundred meters, so that, for example, newly installed VDSL systems generate interferences by cross-talk on existing ADSL lines.
For this reason, within the scope of VDSL standardization (see ITU-Standards G.993.1, G.993.2, G.997, ANSI-Standard T1.424-2004, ETSI-Standard TS101-270), a so-called Downstream Power Back-Off (DPBO) is defined. This is a method for protecting existing ADSL lines. It is the aim of the DPBO to form a transmit power spectral density (TX-PSD) at the VDSL central office side, such that the interference power spectral density caused by a VDSL wire pair on an ADSL subscriber side by far-end cross-talk comprises the same value as the FEXT-PSD generated at the same ADSL subscriber by a neighboring ADSL wire pair.
Also, the currently standardized DPBO assumes a split supply of xDSL-subscribers by a low-data-rate-service on longer lines or cables from an exchange, e.g. ADSL, and by a high-data-rate-service on shorter cables from a cabinet, e.g. VDSL. ADSL- and VDSL-cables are led in parallel in a cable-bundle from the VDSL-cabinet to the xDSL-subscribers. In a useable frequency range of ADSL, VDSL-TX-PSDs are lowered in the VDSL-cabinet insofar that VDSL-cables do not produce more FEXT towards ADSL-subscribers, as the ADSL-lines produce among themselves. The VDSL-DPBO does not need to be performed in frequency ranges not used by ADSL.
However, there are scenarios in which a VDSL is provided from two distinct VDSL-COs. On the one hand, there might be a first line network terminal point DSLAM, i.e. APL-DSLAM (DSLAM=Digital Subscriber Loop Access Module), close to VDSL-subscribers, typically in the basement of a multiple dwelling unit (MDU), such as an office or an apartment building, and secondly, there might be a further distant second VDSL-CO or VDSL-cabinet. A line network terminal point (APL) thereby typically provides copper wire-pairs from the basement to xDSL-subscribers in the dwelling units of the MDU. This implies that both, the VDSL-subscribers and the VDSL-COs are located at different locations, respectively. Within the multiple dwelling unit all subscriber lines are usually led in parallel in a cable-bundle and thereby cause mutual cross-talk.
Since the standard UPBO is configured, based on the attenuation measure kl0, the comparatively short lines from VDSL-subscribers to the APL, in the following referred to as APL-lines, would experience a comparatively high back-off. For this reason, the APL-lines would cause less cross talk in the longer lines from VDSL-subscribers to the VDSL-cabinet, in the following referred to as cabinet-lines, than the cabinet-lines would cause mutually themselves. Hence, the standard UPBO acts in an overcompensating manner and does not allow for an optimum use of the transmission capacity of the APL-lines.
For the downstream, the TX-PSDs of the APL DSLAM interfere with both the downstream transmission of the lines from the VDSL-cabinet, as well as with the downstream-transmission from the lines of the ADSL-exchange. Since ADSL-exchange and VDSL-cabinet are located at different distances from the APL, their signals show different attenuations, respectively, and cannot be simultaneously considered by the standard DPBO procedure. A standard DPBO only allows for the consideration of one attenuation in one frequency range.